The performance, or quality, of an audio system is generally evaluated in terms of "distortions." However, the "distortions" of an audio system measured by conventional methods such as a "total harmonic distortion", for example, are typically not consistent with actual human auditory perception. It often happens that a listener judges a sound produced by an audio system having greater "total harmonic distortion" to be less distorted than one having less "total harmonic distortion".
The inventor has been studying the relationship between human auditory perception and harmonic distortion for some years and has found that the numerical measurement of distortion that best approximates human auditory perception is obtained by a weighting of the components of harmonic distortion; that is, by a filtered weighting of the high pass filter having 3rd through 4th order and characterised in that the damping factor thereof is (d=2 or quality factor Q=1/d=0.5), the cut off frequency of each order being approximately 12 times the fundamental frequency being measured. Consequently, the main object of the present invention lies in a primary weighting network comprising a high pass filter for the third through the fourth orders, supplemented by a secondary weighting network provided to yield a distortion figure that truly reflects human auditory faculties and which might be named an "auditory distortion parameter".
The apparatus according to this invention also includes a weighting network for the measurement of loudspeaker subharmonics so that the results obtained will best reflect human auditory faculties.
The key index of the performance of a high fidelity stereo system, particularly for a loudspeaker, is audio quality. Yet until now there has not been proposed, nor is there anywhere available, an ideal method of performance evaluation. The quality of a sound, good or bad, has traditionally been represented by the measurement of THD (Total Harmonic Distortion) and/or IMD (Intermodulation Distortion). However, such THD and/or IMD measurements are at best mere physical measurements without any human auditory meaning whatsoever; this is because the sensitivity of human auditory faculties increases in response to the number of "orders" of harmonic components. Thus the traditional methods of measuring the components of each harmonic, without a preliminary weighting process, will unavoidably lead to results that do not conform to human auditory perception, and the figures obtained therefrom will fail to represent the quality of a sound consistent with human auditory perception.
Recognizing the facts set forth above, human auditory perception was studied with the object of developing a rational method for the evaluation of audio distortions consistent with actual human auditory perception. Such a method is the primary object of the present invention.
A further object of this invention is to develop a method for measuring such spurious noises as buzzes or rattles that are inherent in a loudspeaker, and which arise out of deviations, typically those beyond tolerances, in the production procedure, these spurious noises being an essential item on quality control check lists. Traditionally such checking is done by skilled professional workers listening to the sweeping sinusoidal sound waves produced from a loudspeaker. Obviously, however, measurements or evaluations made by an individual listening through bare ears adds a subjective taint to the results, easily making them subject to such dispute that an objective standard cannot be established and automatic measurement operations cannot be instituted. A careful study of spurious noises, now shows them to be attributed to harmonic distortions of higher orders; the present invention is thus the result of thorough research into human auditory perception as related to the orders of harmonics as illustrated in the accompanying drawings, in particular as adapted to the evaluation of spurious noises produced by high fidelity loudspeakers, and fall naturally into the auditory distortion domain disclosed hereunder.